ASHRAE 241 Turned CO2 Monitoring Into a Cross-System Control Requirement

Sensor Deployment Has Outpaced the Operational Model

Walk through a campus building where the sensor deployment is already done. The CO2 monitors are installed. The dashboard is connected. The data is logging. Many of those sensors are connected to outside air dampers, which is standard demand control ventilation per ASHRAE 62.1.

When the lecture hall fills up this afternoon, that connection will do what it was designed to do: modulate outside air based on a single CO2 reading in the return air duct. What it was not designed to do is account for current occupancy schedules, verify that the Air Handling Unit (AHU) has capacity to deliver the required equivalent clean airflow, or document that a compliant Infection Risk Management Mode (IRMM) response occurred.

Even within its intended scope, single-sensor DCV carries reliability risks. CO2 sensors lose calibration over time and return air duct readings lag actual room conditions by the time air cycles back through the system. The ventilation response the building believes it is making may not reflect what is happening in the space.

That gap has a growing liability attached to it. ASHRAE Standard 241, published in June 2023, now requires buildings to respond to air quality conditions in ways that single-sensor DCV was never designed to support.

Facilities teams managing infrastructure that deployed CO2 and occupancy sensors after 2020 must now bridge the gap between detection and response.

What ASHRAE 241 Actually Requires

ASHRAE Standard 241, “Control of Infectious Aerosols,” builds on the baseline ventilation requirements of ASHRAE 62.1.

ASHRAE 62.1 defines the minimum ventilation rates a building must always provide. ASHRAE 241 defines what a building must do when infectious aerosol risk is elevated. These are different obligations. A building can be fully compliant with 62.1 and still have no capability to meet 241.

The mechanism at the center of ASHRAE 241 is Infection Risk Management Mode (IRMM). The standard defines IRMM as “the mode of operation in which measures to reduce infectious aerosol exposure documented in a building readiness plan are active.” During IRMM, buildings must provide enough equivalent clean airflow, through a combination of ventilation, filtration, and air cleaning, to substantially reduce transmission risk.

IRMM is not a static configuration set once and left in place. It is a dynamic state a building enters in response to conditions: occupancy level, air quality measurements, event schedules, and system capacity. A building that can detect when conditions require IRMM but cannot activate a ventilation response lacks an ASHRAE 241-compliant response capability.

As ASHRAE 241 begins to appear in procurement requirements and institutional health policy documents at universities, hospitals, and other institutions, the question in those documents is now shifting from “do you monitor IAQ?” to “do you respond to it and can you document that response?”

Facilities teams working through compliance reviews are finding that sensor installation completed the measurement requirement and that ASHRAE 241 added a control requirement on top of it.

Why the Integration Gap Persists After the Sensors Are Installed

When a CO2 sensor registers 1,200 ppm in a classroom, something needs to tell the AHU to increase fresh air delivery. That command must travel from the sensor reading, through a logic layer that understands occupancy context and system constraints, to a BMS setpoint. It also must happen in minutes, not after someone reviews a dashboard at the end of the shift.

In most buildings, that automated path stops at 62.1 compliance.

The sensor logs the reading and the BMS modulates the damper, but nothing connects CO2 data, occupancy schedules, AHU capacity, and system state into a single decision. Instead, CO2 data lives in one system while the HVAC setpoint lives in another. Classroom schedules live in a third. Without an intelligence layer that makes these systems legible to each other, the sensor data stays where it is. Adding more or better sensors does not close this gap.

Building automation incumbents address part of this within their own system boundaries. Their platforms can connect to third-party systems, but connection is not unification. Without a shared semantic layer, CO2 data from one vendor and occupancy data from another remain legible only in isolation. The CO2 monitor from one vendor, the scheduling software from another, and the CMMS from a third do not natively communicate with each other inside a closed BMS ecosystem, and no single building system vendor has the scope to change that, because the data that ASHRAE 241 requires lives across boundaries they do not own.

The ASHRAE 241 response decision requires CO2 data, occupancy context, HVAC state, and system capacity gathered simultaneously from systems that were never designed to share data. A BMS vendor can close that loop within the infrastructure it supplied. An Operational AI platform like Willow closes it across building systems regardless of which vendor supplied them. With Willow, for instance, Willow’s Knowledge Graph gives every asset and sensor a shared language, whether it came from the same manufacturer or not.

What Automated Ventilation Response Looks Like in Practice

Georgia Southern University draws occupancy data from four simultaneous sources: the student information system, event scheduling software, Crestron AV across more than 700 classrooms, and Cisco Spaces WiFi sensors. That occupancy signal flows into a unified data model alongside readings from HVAC sensors, energy meters, and the building automation system.

When those data streams are unified, the system can do what no standalone sensor or BMS can do on its own: issue a setpoint command to the air handling unit because it knows a lecture hall is filling up, the CO2 level is rising, and the current ventilation rate is not calibrated for the actual occupant load. The ventilation response happens automatically, in the minutes that matter, based on what is actually happening in the building, not on a schedule.

The research on why this matters extends well beyond regulatory compliance. The Harvard T.H. Chan School of Public Health COGfx Study (Allen et al., Environmental Health Perspectives, 2015) found that cognitive function scores were 61% higher in conditions with enhanced ventilation compared to conventional building conditions. CO2 concentration was independently associated with cognitive performance across multiple domains. For universities and corporate campuses, ventilation quality is a direct input into the performance of every person working and learning in the building.

A building with elevated CO2 and no ASHRAE 241-compliant response capability is a compliance exposure and a building running below its performance baseline. Both are costs the institution is already absorbing every time the sensor reads high and the HVAC does not respond.

How Operational AI Closes the Gap Between Detection and ASHRAE 241 Compliance

The reason building automation incumbents and IAQ point solutions cannot fully solve the ASHRAE 241 response problem comes down to architecture. BMS platforms and fault detection tools each operate within the boundaries of the data they were built to manage, and neither has access to the unified semantic layer that makes CO2 data, occupancy schedules, HVAC configurations, and building system history legible to each other as a single operational picture.

Connecting building systems is a data pipeline problem that BMS vendors have been solving incrementally for years. Giving every asset across every system a shared semantic meaning, so that a CO2 reading, an occupancy event, and a work order can be reasoned across simultaneously, is an ontology problem. That is what the Knowledge Graph solves.

In fact, Willow’s Knowledge Graph is the world’s largest and richest ontology for the built world, a semantic model that gives every building system a shared language so data from one can inform decisions in another. At Dallas-Fort Worth International Airport, Willow unified five separate BMS systems and three separate CMMS systems into a single operational view of 171,000 or more assets, continuously analyzed by 301,000 or more active skill instances across 124,000 or more live data points. The platform reasons across those systems simultaneously because every asset, sensor reading, and operational event speaks the same language inside the Knowledge Graph.

ASHRAE 241 compliance requires exactly this kind of cross-system reasoning. The decision to increase ventilation depends on four inputs simultaneously: occupancy context, air quality data, system capacity, and energy constraints. Willow’s Knowledge Graph reasons across all four inputs in real time, and the platform’s Active Control issues live setpoint commands through the existing BMS. This allows Willow to autonomously adjust ventilation rates based on real-time conditions, every few minutes, without manual intervention and without replacing any infrastructure already in place.

For facilities teams who have completed sensor deployments and are now facing ASHRAE 241 in procurement requirements and institutional health reviews, Willow helps organizations move beyond detection to response.

See This in Practice

Join Willow CEO Bert Van Hoof and IWBI Executive Vice President Jason Hartke, Ph.D. on June 23 as they walk through what closing the sensor-to-response gap looks like in production, across universities, airports, and critical public facilities. Register for the complimentary webinar, From Building Standard to National Mandate.